In most wireless communication systems, the communication channels are time-varying. In order to maintain high throughput and reliability simultaneously over such time-varying channels, forward error correction (FEC) codes can be employed with the use of automatic repeat-request (ARQ) schemes, such as a hybrid ARQ scheme. In a known hybrid ARQ scheme, the receiver uses FEC codes to correct errors and triggers the retransmission when a residual error is detected. A simple hybrid ARQ scheme is Type-I hybrid ARQ, in which the receiver discards the previous transmission and performs the detection and decoding over the retransmitted signal only. Although this scheme conserves the available memory buffer by discarding the previously received packet, it is not an efficient method in terms of throughput.
Two further types of hybrid ARQ schemes of interest in data packet systems are Chase combining and incremental redundancy (IR) hybrid ARQ (i.e., type-II hybrid ARQ). In the Chase combining scheme, the repeated coded sequence is sent to the receiver upon receipt of the ARQ request. The receiver then combines received multiple copies of coded sequence using maximum ratio combining (MRC) for the detection. In type-II hybrid ARQ, additional parity bits are transmitted at each retransmission. The receiver then decodes the received transmission using all received sequences since they belong to one codeword. IR hybrid ARQ improves the throughput performance by providing a coding gain in additional to the diversity gain. Thus, both Chase combining and IR hybrid ARQ schemes provide throughput gain over type-I hybrid ARQ with the expense of additional buffers and complexity at the receiver.
To achieve high throughput efficiency, both Chase combining and IR hybrid ARQ schemes have been adopted in practical wireless systems. These schemes have also been included in recently established wireless standards. Hybrid ARQ can be classified as synchronous and asynchronous. In synchronous hybrid ARQ, the retransmissions are restricted to occur at known time instants. In asynchronous hybrid ARQ, the retransmissions may occur at any time. For asynchronous hybrid ARQ, additional signaling of the ARQ process number is required.
In terms of transmission attributes, the hybrid ARQ can be further classified as adaptive and non-adaptive. In accordance with adaptive hybrid ARQ, the transmitter may change the attributes at each retransmission including the modulation, coding rate, resource block allocation, etc. In accordance with non-adaptive hybrid ARQ, the modulation, coding rate, and the allocated resource block for retransmissions is the same as the original transmission.
Known solutions have considered non-adaptive hybrid ARQ in which the block length and modulation in each retransmission is assumed to be the same as that in the original transmission. Adaptive hybrid ARQ solutions only consider the binary coded system without employing the discrete quadrature amplitude modulation (QAM).
Additionally, non-adaptive IR hybrid ARQ has been investigated for random Gaussian signals, binary random codes, and low-density parity-check (LDPC) codes. Adaptive hybrid ARQ with Gaussian signaling and turbo coded modulations as well as adaptive IR hybrid ARQ for turbo-coded systems have also been investigated. While, adaptive hybrid ARQ with both IR and Chase combining for turbo coded modulation has been considered, in such investigations, the channel state information (CSI) is assumed unknown to the transmitter.
The design of adaptive hybrid ARQ based on known channel information has been investigated, but only based on capacity expressions for bit-interleaved coded modulations, which is not suitable for practical systems.
Accordingly, improved systems and methods for improving throughput in ARQ wireless transmission systems are desirable.